Those o-ring tend to shrink a little over time when the carbs are sitting dry.
The carbs om my cafe had been sitting for a couple of years and when I first opened the fueltap in 2006 I also had a big leak...
I was suspecting this might happen and had placed a lot of rags under the carbs to soak up any drops of fuel that might escape.
However, when I turned the tap off the leak stopped (of course..), and after reopening the tap a few times I could see the leakage was getting smaller and smaller.
After approx 15 minutes from the first opening the leak was completely stopped as the gas made the o-rings swell back to normal size.
No more leaks after that
Might be worth a shot before you tear the carbs apart
Cheers
Dan